The genetic changes that lead to the activation of two human oncogenes, T-24 and Hs242, have been shown to be point mutations affecting the amino acid composition of these oncogene products. Thus, the change has been localized to the 12th position in the case of T24, and 61st position in the case of Hs242. Biochemical organization of several retroviral and human oncogenes has been studied by nucleotide sequence analysis. From the predicted amino acid sequences, antibodies have been prepared and successfully used in the identification of the transforming gene products of these onc genes. Studies were conducted to understand the genetic changes that lead to the maintenance of the malignant state in tumors that results from a "hit-and-run" phenomenon. These studies provided evidence that such changes involve the rearrangement of protooncogenes which, in turn, result in the production of aberrant onc-gene transcripts that might play a crucial role in the maintenance of malignancy.